The efficiency of an exothermic reaction process often depends primarily on the efficiency of removing large amounts of reaction heat evolved. For an exothermic reaction, the reaction rate and the equilibrium conversion factors usually move oppositely with increasing temperature. Thus, raising the temperature will speed up the rate of the forward reaction, but will decrease the maximum attainable conversion. For increased conversion in a reversible exothermic reaction, a high temperature should be used where the system is far from equilibrium to take advantage of the high reaction rate, but as equilibrium is approached, the temperature should be reduced to shift the equilibrium conversion to a more favorable value. An optimal reactor operating curve (or temperature progression) which maximizes reaction rate is a path corresponding to a locus of maximum rates on a temperature-conversion plot. This path generally follows a decreasing temperature profile moving from the reactor inlet to outlet.
In an endothermic reaction, equilibrium conversion and reaction rate both usually increase with temperature. Thus, the optimum reaction temperature is the maximum temperature allowed. Factors which govern an upper temperature limit include materials of construction, product properties, side reactions, and the like.
For an adiabatic-type fixed bed reactor, it is known that the optimal path (or temperature progression) can be most closely approximated using staged beds with a proper gas flow and interchange of heat. The more stages used, the better the approximation. In practice, prior art reaction processes have been limited to two or three stages with the major limitation being the capital costs for multiple reactor vessels and heat exchange equipment.
Fixed bed catalytic processes are further affected from an efficiency point of view by the pressure drop across the bed. It is known that pressure drop depends on the resistance of the flow path which, in turn, is generally proportional to path length and the particle size of the catalyst. Excessive pressure drop can result in channeling through the bed, poor catalyst efficiency and higher compression energy. Reactors with cylindrical geometry can have direct reactant flows through the bed. Axial flows have typically been used, despite having a longer flow path compared to cross-flow, apparently due to the difficulty of axially distributing the reactants (for a transverse flow path) and controlling the temperature progression for maximized results.
Accordingly, there is a need in the art for a reactor design which can implement a flow regime for reduced pressure drop and an optimized temperature progression for enhanced reaction rate without excessive capital expense.
U. S. Pat. Nos. 2,276,307 to Houdry and 2,432,543 to Prickett et al. describe catalytic converters comprising tubes disposed throughout a bed of catalyst material in order that a heat exchange fluid can be circulated indirectly through the bed.
U. S. Pat. No. 2,989,383 to Miller describes an adsorption apparatus having a plate heat exchanger assembly embedded in the adsorbent bed for controlling the temperature uniformity of the bed.